406 THE AMERICAN NATURALIST.  [VOL. XXXIII. 
MopERN DEPOSITS OF THE PLAINS. 
There are two kinds of deposition now going on in this 
region: 
I. River and Flood Plain Sediments. — Sands and clays are 
deposited in the valleys whenever continental deformation 
causes a decrease in the fall of the river bed. Mr. Gilbert’s 
clear exposition of this mode of sedimentation, as exemplified 
in the Arkansas valley, makes further comment superfluous.! 
II. Prairie Loess. —Wind erosion (deflation), very active in 
this arid region, carries off from all exposed rock or soil a large 
amount of material, most of which is deposited on the sodded, 
or partially sodded, prairie surface. The deposit is heavier in 
hollows, where temporary “lagoons” or denser grass occur, and 
tends to produce an extremely level and uniform surface. This 
deposition through the Pleistocene has produced a fine unstrat- 
ified, extremely uniform covering of loam (loess or marl) over 
the greater part of the plains; I have seen it in places two 
hundred feet thick. It was formerly supposed to be a lacus- 
trine sediment, and that the rivers subsequently cut their 
channels in the floor of the dried-up lake. It is now, I believe, 
generally considered zolian, and a significant corollary of the 
mode of deposition, outlined above, is that it was built up at 
the sides of the river valleys, deposition in the valleys being 
checked by erosion; the valleys, therefore, are older than the 
plains. : 
Practically the only fossils found in the loess are the bones of 
land animals. No plant remains occur; the prolonged exposure 
to water and air permits the nearly complete oxidation of vege- 
table matter. Near the surface are peaty layers and land shells, 
but the conditions of burial do not favor their fossilization. 
Dark lines mark the position of lagoons on the former prairie 
surface, and slight changes in the quality or fineness of the 
deposit produce a bedded appearance, uniform over large areas. 
The extent and uniformity of this Prairie Loess are especially 
notable. It is the prevalent surface deposit over half a dozen 
western states in the greater part of the plains region. 
1 U.S. Geol. Survey, XVIIth Ann. Rep., Pt. ii, P: 575: 
